How do I delete notifications?

I’m getting quite a collection of notifications there. I see that it keeps track of the ones I’ve responded to and the ones I haven’t – but the list just keeps getting longer and longer. How do I delete notifications once I don’t need them any more?

Disco Nanny Bot says: Your topic is similar to…
Do MRI-manufacturers make a large profit on MRI machines?

Same, and I only noticed an “archive” button for the messages after poking around. Is this the only option to make them ‘go away’?

I tried to delete mine; wound up on my notifications page, but there seems to be no way to select messages to delete. There’s a “delete all” box, but it’s greyed out, and clicking on it does nothing.

My preferences gives an option to disable notifications entirely; is that the only option I’ve got? I did do that for now; but that didn’t change anything about the existing list. And I’m not sure that I just want to leave them turned off.

As far as I know, they’re there forever. I just ignore them unless I’m looking for something that I know I got a notification about.